The present invention relates to the field of lubricants and protective coatings. More particularly, the invention relates to lubricants and protective coatings incorporating fluoropolymer particles.
Fluoropolymers, such as polytetrafluoroethylene, have gained widespread acceptance for reducing surface friction and as protective coatings. In particular, polytetrafluoroethylene (PTFE) has been used on various types of surfaces from the familiar frying pan to rubbing parts and valves in complex apparatus, such as artificial human hearts. Unfortunately however, the cost of either making entire parts from PTFE or applying PTFE coatings to existing surfaces is relatively expensive.
Considerable research has been undertaken involving the incorporation of minute solid fluoropolymer particles in liquid lubricants to thereby achieve some of the benefits of the fluoropolymers in a liquid medium. In particular, several engine oil additives which include dispersed particles of fluoropolymers are disclosed in the patent literature. For example, U.S. Pat. No. 3,933,656 to Reick, teaches a modified lubricant for an internal combustion engine which comprises a major amount of a conventional motor oil, with a minor amount of sub-micron size PTFE particles, and a neutralizing agent to stabilize the dispersion to prevent agglomeration and coagulation of the particles. In this and several other patents, the theory is proposed that the lubricating action of the PTFE containing lubricant is enhanced by virtue of the fact that the PTFE particles somehow become attached to the surfaces of the engine thus lubricated, thereby creating a renewable coating of PTFE.
Naturally, substantial effort has been expended in this field to optimize the effectiveness of these PTFE containing lubricants as well as to minimize the problems associated therewith. In particular, much has been done to prevent the otherwise inherent problems of agglomeration, coagulation, and settling of the PTFE particles. For example, British Patent Application No. 2,090,284 teaches the method of coating the PTFE particles with a "buoyant" carrier with a relatively low specific gravity, after which the coated PTFE particles are dispersed in a heavier oil, i.e. one with a higher specific gravity such as mineral oil. In this way, the PTFE particles which have an even higher specific gravity are "floated" in the oil. For examples of other methods of obtaining a dispersion of PTFE particles in a carrier lubricant see U.S. Pat. Nos. 4,127,491; and 4,396,514.
One limitation with many of the PTFE containing lubricants involves the fact that the oils used to achieve and maintain good dispersion of the particles have been of relatively high viscosity. These high viscosity oils, although well suited for applications such as in crankcases for automobiles, are undesirable for other applications such as for lubricating weapons or for coating exposed surfaces for corrosion protection. Also, the high viscosity oils are difficult if not impossible to apply in a spray format, particularly without the use of a high pressure aerosol container. Furthermore, the higher viscosity oils display a more limited amount of surface penetration.
U.S. Pat. No. 4,333,840 to Reick, discloses a "hybrid PTFE lubricant" wherein the viscosity of a PTFE containing lubricant was lowered by blending with the original PTFE oil a second oil of lower viscosity. This patent recites the advantage that the lower viscosity PTFE oil is better suited for use with weapons, particularly as a lubricant and a means to prevent fouling.
Although the hybrid PTFE oil disclosed by Reick has a lower viscosity and may therefore be better suited for certain applications such as weapons, it also faces certain limitations. In particular, because the oil is now of a lower total viscosity, its retention on surfaces will likely be limited. That is, the lighter oil carrier for the PTFE tends to be more easily removed from the surfaces to which it is applied.